Tendon anchorage



June 17, 1969 G H. HOWLETT TENDON ANCHORAGE Sheet of 3 Original Filed Feb. 24, 1965 mmm llllt IN VENTOR.

WARREN, RUBIN, BRUCKER g. cmcxnnmc ATTORNEYS June 17, 1969 G. H. HOWLETT 3,449,876

TENDON ANCHORAGE Original Filed Feb. 24, 1965 Sheet 3 of s FIG.2

4 m I I M WN'TWENTOR. Fl G 3 George H. Howler! WARREN, RUBIN, BRUCKER 6 CHICKERING ATTORNEYS June 17, 1969 a. H. HOWLETT 3,449,376

- TENDON ANCHORAGE Original Filed Feb. 24, 1965 Sheet 3 INVENTOR. George H. Howlerr.

WARREN, RUBIN, BRUCKER 5 CHICKERING ATTORNEYS United States Patent 3,449,876 TENDON ANCHORAGE George H. Howlett, Oakland, Calif. (746 Folger Ave., Berkeley, Calif. 94710) Continuation of application Ser. No. 434,978, Feb. 24, 1965. This application Feb. 21, 1968, Ser. No. 707,067

Int. Cl. E04c /01, 5/08; F16g 11/05 U.S. Cl. 52-230 7 Claims ABSTRACT OF THE DISCLOSURE A tendon anchorage for use in connection with pretensioning and post-tensioning concrete is disclosed which includes an anchor member formed with a bore with a plurality of wedges mounted therein and the wedges are designed to receive pairs of tendons between their adjacent side faces such that tensioning of the tendons causes said wedges to apply a clamping force which is self-adjusting between each pair of tendons whereby the tendons are secured in the anchorage with a substantially equal force which is proportional to the tension applied to the tendons. In addition, the anchorage may include interfitting convergent cam planes for inducing the clamping forces in the wedges.

This is a continuation application of application, Ser.

No. 434,978, filed Feb. 24, 1965, now abandoned. The 0 present invention relates to anchorage devices for tendons, bar, wire and cable, in structures wherein the anchorages are [designed to hold near or up to the ultimate strength of the tendons.

Tendon anchorages fall into several general types depending upon the type of tendon used. For example, bar tendons may be anchored by threading the tendon or by the use of wedges. Wire tendons are not adapted for threading and wedges are most commonly used for anchoring wire tendons. In the case of cables, the wire ends may be splayed out and secured in a poured socket which may be filled with zinc or similar material forming a relatively large casting. Threading weakens the tendon because of reduction in cross sectional area and also because any notching of the tendon creates stress raisers. In the case of wedges, it is highly desirable that the wedges be automatically self-setting so as to increase their clamping force on the tendon as the tensional forces on the tendon are increased and that each tendon be clamped or gripped with substantially the same gripping force. In order to obtain such a self actuation some anchorages have provided the jaw faces of the wedges with sharpened teeth or serrations which are capable of biting into the tendon. Such teeth or serrations like the threads are undesirable in creating stress raisers at the point of contact.

Another serious problem encountered with multiple wedge tendon anchorages of the prior art occurs when the load on the tendons rapidly changes from a high tension load to a compressive load. This load reversal phenomenon occurred, for example, in many prestressed structures during the Alaskan earthquake. Tendons, once under high tension loads, momentarily went slack causing the tendon gripping wedges to fly backwards out of the anchorages, and when the load reversed again, the anchorages failed completely.

Accordingly, it is an object of the present invention to provide a multiple tendon anchorage which will be self adjusting to positively clamp all of the ten-dons in the anchorage with equal force and provide for equal tensioning of the tendons and to accomplish the foregoing in a device of minimum size and composed of a minimum number of durable parts capable of permanent troublefree service.

A still further object of the present invention is to provide a tendon anchorage in which the tendons may be tensioned, tested and re-tensioned without destroying or replacing the anchorage or any of the parts thereof.

Another object of the present invention is to provide a multiple tendon anchorage which affords an easy and simultaneous tensioning of the tendons.

Still another object of the present invention is to provide a multiple tendon anchorage of compact design which is safer and more reliable when installed and easier to assemble in the field.

The invention possesses other objects and features of advantage, some of which of the foregoing will be set forth in the following description of the preferred form of the invention, which is illustrated in the drawings accompanying and forming part of this specification. It is to be understood, however, that variations in the showing made by the drawings and description may be adopted within the scope of the invention as set forth in the claims.

Referring to said drawings (three sheets):

FIGURE 1 is a cross sectional view of the tendon anchorage constructed in accordance with the present invention and shown in conjunction with a structural mem- FIGURE 2 is a perspective view of the anchorage shown in FIGURE 1.

FIGURE 3 is a cross sectional view of the anchorage shown in FIGURES 1 and 2.

FIGURE 4 is a cross sectional view of the anchorage shown in FIGURES l, 2, and 3 and is taken substantially on the plane of line 44 of FIGURE 3.

FIGURE 5 is a cross sectional view similar to FIGURE 1 but showing a modified form of the anchorage of the present invention.

FIGURE 6 is a perspective view of a portion of the anchorage illustrated in FIGURE 5.

FIGURE 7 is a cross sectional view portion shown in FIGURE 6.

FIGURE 8 is a cross sectional view of the anchorage portion shown in FIGURES 6 and 7 and is taken substantially on the plane of line 8-8 of FIGURE 7.

FIGURE 9 is a cross sectional view of an anchorage portion constructed in accordance with FIGURES 7 and 8 and further formed with pairs of tendon gripping recesses.

The tendon anchorage of the present invention comprising an anchor member 16a having a wall defining a frusto-conical bore 17a dimensioned at the small diameter end 18a thereof to receive a plurality of tendons 42 and 43 to be tensioned, a plurality of wedges 22a mounted in the bore forming a frusto-conical assembly therein with the peripheral sides 26a of the wedges engaged with the bore wall and the side faces 24a of the wedges being mounted in spaced opposed relation for receiving and clamping tendons therebetween, the opposed side faces of adjacent wedges being formed with laterally spaced pairs of opposed longitudinally extending recesses 36a and 361) each formed and dimensioned to receive and engage partially around the periphery of one of the tendons, the peripheral sides of the wedges having substantially the same pitch as the bore wall for mated sliding engagement therewith whereby upon tensioning of the tendons wedges 22a are drawn in the direction of small diameter bore end and thus into clamping engagement with the tendons. The clamping force generated is proportional to the number of tendons secured by wedges 22a and the aforesaid pitch of the wedges and bore wall and the area of contact between the wedges and tendons. These parameters are selected to obtain a positive clamping of the of the anchorage tendons up to their ultimate tensile strength without substantial damage to the tendons; and the anchor member 16a is dimensioned and constructed to supply the requisite banding force.

The anchorage is illustrated in FIGURE 1 is applied to the post-tensioning of a structural member 27, such as a concrete beam, although, as will be understood, the anchorage device of the present invention may be used for positively securing and anchoring a plurality of tendons in substantially any structure including cable and bridge anchorages and the like. When applied to the posttensioning of a concrete beam as illustrated in "FIGURE 1, the anchorage member 16a is provided with a face 28, adjacent the small diameter end 18a of bore 17', for engaging an abutment '29 of the structural member; and the concrete member is sometimes formed with a tunnel 31 in which the plurality of tendons 42 and 43 may be placed and supported by longitudinally spaced separators 32.

Anchor member 16a is preferably made of steel plate of sufficient thickness to withstand the bending loads imposed and also substantially corresponding in thickness to the length of the wedges 22a, which in turn determine the area of contact between the wedges and tendons. Some unsupported overhang of the wedges at the opposite sides of the anchor member is permissible. Because of the very high order of clamping forces developed by the present multiple wedge anchorage construction, the anchor member must have a diameter or circumferential mass surrounding the bore of suflicient radial dimension to contain and withstand the extremely high radial forces to which the member is subjected as a reaction to the very high order of clamping force. This strength to prevent bursting of the anchor member is sometimes referred to as banding strength or force.

The configuration of the wedges in contact with the tendons may be adapted to suit particular tendons and installations, it being sufficient that there be provided enough area of cotnact to provide automatic actuation of the wedges and positive anchoring of the tendons. For example, the opposite sides 23a and 24a of the wedges may be flat and yet provide in proper designs for adequate gripping of round bars or wires. Where fiat sided tendons are used, flat sides on the wedges are preferred. The area of contact between the sides of the wedges and the tendons, in the case of circular tendons, may be substantially increased by forming at least one of the opposed radial side faces 23a and 24a of the wedges with a pair of longitudinally extending recesses or troughs 36a and 36b, which are each formed and dimensioned to receive and to engage patrially around the periphery of one of the tendons so as to increase the area of contact therewith. This effect may be maximized by forming opposed substantially semi-circular recesses in both of the opposed walls 23a and 24a as shown in FIGURE 3 so that the wedges fit around opposite sides of the tendons. A similar effect can be obtained by making the wedges out of somewhat softer material than the tendons so that the wedges will be caused to cold flow into greater contact with the tendons as the high order of magnitude of clamping force is exerted. Another alternate method of increasing the area of contact between the tendons and the wedges is to make the wedges out of a somewhat harder material than the tendons so as to produce a cold flow of the tendons into conformity with the shape of the wedges under the influence of the high order of magnitude of clamping forces used. In any such designs, the diameters of the tendons and the depths of the recesses or troughs or the amount of cold flow of the wedges or tendons are predetermined so that all of the circumferential spaces between the adjacent wedges is not taken up during the clamping action, but on the contrary some space is left between adjacent wedge faces 23a and 24a when the tendons are anchored.

Because of the automatic self adjusting characteristic of the wedges and the cold flow conformity of the wedges and tendons, manufacturing tolerances in the production of the parts of the present anchorage are easy to maintain. Also because of the foregoing characteristics, it is quite feasible to anchor different types of tendons, for example, stranded cable and bars in the same anchorage, intermixing the different types of tendons in the circumferential assemblage.

As herein above noted, the clamping force on the tendons secured in the anchorage. The number of tendons is the number of tendons and the total tensioning force exerted by them. Consequently increased clamping force may be obtained by increasing the total number of tendons secured in the anchorage. The number of tendons is conveniently increased in the present apparatus by mounting a plurality of tendons 42 and 43, as shown in FIG- URES 1 to 4, between the opposed faces 2311 and 24a of each adjacent pair of wedges 22a. As here shown, the two sets of such troughs 36a and 36b are disposed in concentric circles in relation to the axis of the bore of the anchor member. As will be understood, any number or tendons may be mounted between the opposed faces 23a and 24a of the adjacent wedges and the automatic selfadjusting action of the wedges will ensure equal clamping of all of them.

Another embodiment of the present invention is shown in FIGURES 5 to 9 wherein the anchorage is designed to permit taking up or the tensioning of the tendons, testing of the strain on the tendons after a given period of time, and retention of the wedges within the anchor member upon reversals in the tendon loading. In this embodiment, the anchor member 161) and the plurality of wedges 22b are formed with a pluarlity of longitudinally spaced annular interfitting cam planes 46 and 47 on the interior wall of the bore of the anchor member and on the exterior of the wedges. As will be observed from the drawings, these cam planes diverge away from the direction of force exerted by the tendons so as to translate the sum total of axial forces of the tendons into transverse clamping forces exerted by the side faces 23b and 24b against the tendons 14b positioned therebetween. As a further important feature of this form of the invention, the cam planes 46 and 47 are spirally formed to provide joint threading of the wedge members 22b into and out of the bore of the anchorage nut member 16b. The plurality of threaded cam planes provides a more compact structure where this is desirable and also permits easy relative rotation of the exterior nut member 1612 for taking up of play or free space where such action is desirable. Axially extending troughs or recesses 36b may be formed in either one or both of the side faces 23b and 24b, as in the above embodiment, for the securing of one or more tendons between each opposed pair of faces 23b and 24b.

It is also an important feature of this form of the in vention that cam planes 46 and 47 are interconnected by at least one surface 40 and mating surface 4.1, formed in the wedges and bore wall respectively, so as to limit axial displacement of wedges 22b in the direction of divergence of the bore. Spirally formed cams achieve this end and in addition a series of longitudinally spaced diverging cam surfaces which are circular, not a continuous spiral cam surface, would also sufiice. Such a series of circular cams (not shown in the drawing) can be formed exactly as shown in FIGURES 5 to 9 with the exception that the cam lands would not be interconnected. An anchorage having circular or spiral cam planes formed in the peripheral sides has the substantial advantage of being formed in a manner to be positively retained by anchor member 16b against being urged out of the anchorage in the direction of divergence of the cam planes. This is an important safety feature of the anchorage of the present invention for applications where a stress reversal or reversal of loading is possible. Spiral or circular cam planes, constructed as illustrated and described herein, will not be thrown out of the anchor member if tendon loading is momentarily released, although the clamping action of wedges 22b will tend to be released. When the tendons again are loaded, wedges 22b will reclamp the tendons, and although as re-clamped the anchorage will not grip the tendons at the axialload prior the load reversal, the tendons will not pull free and cause the entire structural member fail. Thus, the structural member must be replaced or retensioned, but it maintains the load long enough after the reversal to allow evacuation of property or personnel endangered by the weakened structure. 7

Both spiral and circular cam constructions have the additional assembly advantage of allowing the tendons to be inserted into recesses 36b under the field assembly conditions normally encountered without forcing the wedges backwards and out of member 16b. Moreover, circular cam planes also afford a simultaneous take-up during tensioning by sliding the wedges together axially along the tendons.

When circular cam planes are formed on the wedge peripheral surfaces, the wedge assembly obviously cannot be rotated into the anchor member to be assembled. Instead wedges must be individually inserted into member 16b and urged up into the mating recesses in the anchor member and rotated or clustered together to allow later wedges to be inserted. The depth of the recesses and the spaces between the adjacent wedges must be selected to allow the wedges to be urged into abutting relationship and the last wedge to be inserted.

In FIGURE 9, a wedge assembly having wedges 220 formed to receive and clamp pairs of tendons 42 and 43 is illustrated. The periphery of wedges 220 are formed with spiral cam lands 47 suitable for use with a nut member 16b in a manner as shown and described in connection with FIGURES to 8. Thus, the advantages of gripping pairs of tendons and diverging cam plane can be enjoyed.

Various hydraulic jacking units are available for applying tension to the tendons. A typical layout is illustrated in FIGURE 1 in which a hydraulic jack 51 1s mounted between a jacking head 52 and a jack chair 48. The hydraulic jack 51 is formed with a central open core for surrounding the tendons 42 and 43 and to bear against the jacking head 52 concentrically of the tendons. Similarly jack chair 48 is provided with spaced legs 49 and 50 which bear upon the anchor member 16a on opposite sides of the conical assembly of wedges 22a. Jacking head 52 is here formed as a typical anchorage of the type herein above described having a plurality of frustoconical wedges 53 mounted on opposite sides of each of the tendons and with their tapered outersides 54 in sliding engagement with an internal frusto-conical wall 60. As will be observed the small diameter end of the wedge assembly is adjacent the hydraulic jack so that the wedges will move automatically into clamping engagement with the tendons as the hydraulic jack applies axial force to the jacking head as indicated by the arrows in FIGURE 1.

The integrated beam structure illustrated in FIGURE 1 includes another anchorage at the left hand side of the figure for securing the tendons 42 and 43 at that side of the beam. This anchorage is again similar in all respects to anchorages herein above described and includes an anchor member 57 which is mounted against an abutment 56 of the beam and which is formed with a frustoconical bore .as defined by internal wall 61 for receiving therethrough the plurality of tendons 42 and 43; and a plurality of segmental frusto-conical wedges 59 which are mounted between the tendons and have their exterior tapered surfaces 58 in mated sliding engagement on wall 61. As will be observed from FIGURE 1, the small diameter end of the wedge assembly faces in the direction of the tensioning of the tendons so that the latter will be automatically anchored by wedges 59 as tension is applied by the hydraulic jack. As will be further observed from FIGURE 1, tensioning of the tendons by the hydraulic jack will be accompanied by a displacement of the tendons at anchor member 16a in a direction loosening wedges 22a. When the tendons are brought up to desired tension, wedges 22a are reset to the left, as seen in the view and as indicated by arrows 62, .so that upon releasing of the force applied to the jacking head, wedges 22a will move automatically to anchor the tendons in tensioned position.

The use of .a nut anchorage construction as illustrated in FIGURE 5 makes possible the tensioning and re-tensioning of the tendons so as to take up for tensioning loss which accompanies the slight movement of the Wedges to their setting position. In this arrangement, the left hand end of tendons 19b are anchored in the manner illustrated in FIGURE 1. As hydraulic jack 51b applies its expansion force to jacking head 52b and chair 43b, nut member 16b will be carried to the right, as seen in the figure in slightly spaced relation to anchor member 16b, which is mounted with its face 28b against abutment 29b provided by beam 27b. When the tendons are fully tensioned, nut body 16b may be rotated so as to move axially to bear with its end face 66 against the adjacent end face 67 of the anchor member 16b to thereby initially set wedges 22b for automatic anchoring of the tendons in place when the jacking tension is released.

At any time thereafter the tensioning of the tendons may be tested without destroying or replacing the anchorage by simply reinstalling hydraulic jack 51b, jacking head 52 and chair 48b; tensioning the tendons while observing a pressure gauge on the hydraulic jack; and testing the anchor body 16b until it may be rotated. Reading the pressure gauge on the hydraulic jack at the moment the anchor body 16b is initially rotated indicates the magnitude of the preset tension on the tendons. After the tendons have been re-tensioned, the nut body 16b may be run down against face 67 to positively anchor the tendons in their newly tensioned condition upon release of the jacking force.

The present anchorage is capable of producing clamping forces exceeding the ultimate strength of the steel used in the tendons and wedges. By the addition of tendons to the assembly thereby increasing the total aggregate pull on the wedge assembly, this gripping or clamping force can be increased to the point of squeezing or extruding either the tendons or the wedges whichever will flow first. A similar effect may be obtained by decreasing the angle of taper or pitch of the frusto-conical wall and wedges. On the other hand, the likelihood of deforming the tendons or wedges may be counteracted by increasing the area of contact between the wedges and tendons as for example the provision of the longitudinal troughs or recesses 36 or the increasing of the length of the tendon. The three principal factors in the design of the anchorage are therefore the number of tendons which are to be anchored; the pitch of the wedges and bore wall; and the area of contact between the wedges and the tendons, it being understood that in all cases the design should provide free sliding engagement between the wedges and bore wall with minimum friction therebetween, and adequate, and in some instances maximum, frictional engagement between the wedges and tendons. These factors are adjusted and proportioned to fit individual tendon anchorage arrangements so as to obtain positive clamping of the tendons up to their ultimate strength without substantial damage to the tendons.

By way of example, a satisfactory design in the case of thirty-two tendon assembly illustrated in FIGURES 1 to 4 using A-inch wire tendons having an individual tensile strength of 10,000 to 11,000 pounds providing an aggregate tensile force of 320,000 to 352,000 pounds may have the following parameters:

Wedge pitches in the range of 10 to 15. Wedge length in the range of 2" to 2 /2". Anchor member a 10" x 10" 7 plate, 2" thick composed of C-1040 steel. Space between wedge faces at gripping strength to inch.

The aforementioned dimensions are given to purposes of illustration and designers may change these to suit individual installations. For example, the pitch of the wedges and bores may vary widely depending upon the number of tendons used from about to 20 or more. In the case of large cables, as used for example in a cable suspension bridge, the ends of the large main cable may be splayed out at the anchorages into cluster groupings with each cluster resembling one of the embodiments of the present invention, as for example the multi-Wire embodiment of FIGURES 1 to 4. There are conditions calling for a dead end anchorage wherein it may be desirable to flare the wires out as in the present Freyssinet anchorage. In such case, the individual wire troughs will be placed on an angle to the axis, out of parallel, and the wires may be grouped closer together at the point where they go into a beam or the like where for example they may touch one another. When an anchorage is employed which incorporates inclined cam planes, as illustrated in FIGURES 5 to 9, the cam planes may be formed with a pitch as high as with 22 being preferable on an eight tendon anchorage having wedges from 2 to 3 inches in length. As above set forth, the pitch is related to the number of tendons anchored and the area of the wedge which engages the tendon.

I claim:

1. A tendon anchorage comprising an anchor member having a wall defining a frusto-conical bore dimensioned at the small diameter end thereof to receive a plurality of tendons to be tensioned, a plurality of wedges mounted in said bore forming a frusto-conical assembly therein with the peripheral sides of each of said wedges engaged with said bore wall and the side faces of each of said wedges being mounted in spaced opposed relation for receiving and clamping tendons therebetween, said opposed side faces of adjacent wedges being formed with laterally spaced pairs of opposed longitudinally extending recesses, each formed and dimensioned to receive and engage partially around the periphery of one of said tendons, said peripheral sides of said wedges having substantially the same pitch as said bore wall for mated sliding engagement therewith whereby upon tensioning of said tendons said wedges are drawn in the direction of said small diameter bore end and thus into clamping engagement With said tendons with a clamping force proportional to the number of tendons secured by said wedges and the aforesaid pitch of said wedges and bore wall and the area of contact between said wedges and tendons.

2. A tendon anchorage as defined in claim 1, wherein said side faces are formed to extend radially from said peripheral sides toward the longitudinal axis of said bore.

3. A tendon anchorage comprising, an anchor member having a bore dimensioned to receive therethrough a plurality of tendons to be tensioned, a plurality of wedges mounted in said bore and formed to receive a plurality of tendons with each of the tendons disposed between opposed side faces of pairs of adjacent wedges disposed for clamping against the opposite sides of the tendons, the peripheral sides of said wedges and the wall of said bore being formed with a plurality of longitudinally spaced annular interfitting cam planes diverging away from the direction of force exerted by said tendons and inclined to the axis of said bore so as to translate the sum total of axial forces of said tendons into transverse clamping forces exerted by said side faces against said tendons.

4. A tendon anchorage as defined in claim 3, wherein said cam planes are spirally formed to provide joint threading of said wedges axially of said bore to afford, upon relative rotary displacement of said member, a simultaneous and equal take-up of said tendons and wedges.

5. A tendon anchorage as defined in claim 3, wherein at least one pair of said opposed pairs of side faces of said wedges is formed with two longitudinally extending recesses formed and dimensioned to receive and to engage partially around the periphery of one of said tendons to increase the area of contact therewith.

6. A tendon anchorage as defined in claim 3, wherein said side faces are formed to extend radially from said peripheral sides toward the longitudinal axis of said bore.

7. A tendon anchorage as defined in claim 3 wherein said interfitting cam planes are connected by at least one pair of interfitting surfaces formed in the Wall defining said bore and said peripheral sides to extend into said bore and limit axial displacement of said wedges in the direction of divergence of said cam planes formed in said bore.

References Cited UNITED STATES PATENTS 2,945,720 7/1960 Osmun 24263.5 3,123,879 3/1964 Boduroff et al 24-122.6 3,343,808 9/1967 Howlett 24136 FOREIGN PATENTS 265,844 4/ 1964 Australia.

HENRY C. SUTHERLAND, Primary Examiner.

JAMES L. RIDGILL, JR., Assistant Examiner.

US. Cl. X.R 

